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I am trying to sum the unit value by date, and create a new array where there are no duplicate dates. For example, I want to calculate the total of 2015-12-04 00:01:00. This date has occurred 2 times in the following data, its value is 5 and 6, which is going to be:

[{date: '2015-12-04 00:01:00', unit: 11}, ... etc]

I have tried arr = results.map(x => x.unit).reduce((a,c) => a + c) but it only return a single value, not an array.

results = [ { unit: 5, date: '2015-12-04 00:01:00' },
          { unit: 10, date: '2015-12-04 00:01:00' },
          { unit: 5, date: '2015-12-04 00:31:00' },
          { unit: 9, date: '2015-12-04 00:31:00' },
          { unit: 5, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:31:00' },
          { unit: 5, date: '2015-12-04 01:31:00' },
          { unit: 10, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:31:00' },
          { unit: 9, date: '2015-12-04 02:31:00' },
          { unit: 5, date: '2015-12-04 03:01:00' },
          { unit: 9, date: '2015-12-04 03:01:00' },
          { unit: 5, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 04:01:00' },
          { unit: 5, date: '2015-12-04 04:01:00' }];

arr = results.map(x => x.unit).reduce((a,c) => a + c);
console.log(arr);

I am trying to sum the unit value by date, and create a new array where there are no duplicate dates. For example, I want to calculate the total of 2015-12-04 00:01:00. This date has occurred 2 times in the following data, its value is 5 and 6, which is going to be:

[{date: '2015-12-04 00:01:00', unit: 11}, ... etc]

I have tried arr = results.map(x => x.unit).reduce((a,c) => a + c) but it only return a single value, not an array.

results = [ { unit: 5, date: '2015-12-04 00:01:00' },
          { unit: 10, date: '2015-12-04 00:01:00' },
          { unit: 5, date: '2015-12-04 00:31:00' },
          { unit: 9, date: '2015-12-04 00:31:00' },
          { unit: 5, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:31:00' },
          { unit: 5, date: '2015-12-04 01:31:00' },
          { unit: 10, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:31:00' },
          { unit: 9, date: '2015-12-04 02:31:00' },
          { unit: 5, date: '2015-12-04 03:01:00' },
          { unit: 9, date: '2015-12-04 03:01:00' },
          { unit: 5, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 04:01:00' },
          { unit: 5, date: '2015-12-04 04:01:00' }];

arr = results.map(x => x.unit).reduce((a,c) => a + c);
console.log(arr);

• javascript
• arrays
• dictionary
• ecmascript-6
• reduce

Share Improve this question Follow edited Dec 13, 2018 at 3:20 Randy Casburn 14.2k11 gold badge1919 silver badges3232 bronze badges asked Dec 13, 2018 at 3:14 Anson CAnson C 50777 silver badges1818 bronze badges 2

• 1 So you're trying to sum the amount of a certain date you have? – Andrew Li Commented Dec 13, 2018 at 3:16
• Yes, the sum of each date – Anson C Commented Dec 13, 2018 at 3:21

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 5

You can reduce your data to a dictionary and then map it to an array like so

results = [ { unit: 5, date: '2015-12-04 00:01:00' },
      { unit: 10, date: '2015-12-04 00:01:00' },
      { unit: 5, date: '2015-12-04 00:31:00' },
      { unit: 9, date: '2015-12-04 00:31:00' },
      { unit: 5, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:31:00' },
      { unit: 5, date: '2015-12-04 01:31:00' },
      { unit: 10, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:31:00' },
      { unit: 9, date: '2015-12-04 02:31:00' },
      { unit: 5, date: '2015-12-04 03:01:00' },
      { unit: 9, date: '2015-12-04 03:01:00' },
      { unit: 5, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 04:01:00' },
      { unit: 5, date: '2015-12-04 04:01:00' }]

// first make an object with {date: unit} value as such
const newResults = results.reduce((acc, item) => ({
  ...acc,
  [item.date]: (acc[item.date] || 0) + item.unit
}) , {})

console.log(newResults)
/*
{
  "2015-12-04 00:01:00": 15,
  "2015-12-04 00:31:00": 14,
  "2015-12-04 01:01:00": 15,
  "2015-12-04 01:31:00": 15,
  "2015-12-04 02:01:00": 15,
  "2015-12-04 02:31:00": 14,
  "2015-12-04 03:01:00": 14,
  "2015-12-04 03:31:00": 15,
  "2015-12-04 04:01:00": 15
}
*/

// now if you want array representation for this you can do

const finalResult = Object.keys(newResults).map(key => ({date: key, unit: newResults[key]}))

console.log(finalResult)
/*
[
  {
    "date": "2015-12-04 00:01:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 00:31:00",
    "unit": 14
  },
  {
    "date": "2015-12-04 01:01:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 01:31:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 02:01:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 02:31:00",
    "unit": 14
  },
  {
    "date": "2015-12-04 03:01:00",
    "unit": 14
  },
  {
    "date": "2015-12-04 03:31:00",
    "unit": 15
  },
  {
    "date": "2015-12-04 04:01:00",
    "unit": 15
  }
]
*/

You need to make the accumulator in reduce an object whose keys are the dates. Then you can accumulate in the units property.

You can use Object.values() to convert this back to an array.

results = [ { unit: 5, date: '2015-12-04 00:01:00' },
          { unit: 10, date: '2015-12-04 00:01:00' },
          { unit: 5, date: '2015-12-04 00:31:00' },
          { unit: 9, date: '2015-12-04 00:31:00' },
          { unit: 5, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:01:00' },
          { unit: 10, date: '2015-12-04 01:31:00' },
          { unit: 5, date: '2015-12-04 01:31:00' },
          { unit: 10, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:01:00' },
          { unit: 5, date: '2015-12-04 02:31:00' },
          { unit: 9, date: '2015-12-04 02:31:00' },
          { unit: 5, date: '2015-12-04 03:01:00' },
          { unit: 9, date: '2015-12-04 03:01:00' },
          { unit: 5, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 03:31:00' },
          { unit: 10, date: '2015-12-04 04:01:00' },
          { unit: 5, date: '2015-12-04 04:01:00' }];

arr = Object.values(results.reduce((a, {
  unit,
  date
}) => (a[date] = {
  date: date,
  unit: a[date] ? a[date].unit + unit : unit
}, a), {}));
console.log(arr);

you should group by date in a new element and then create a new array with the sum of units

let indexedDates = {};
results.forEach(result => {
   let arrayByDate = indexedDates[result.date] || [];
   arrayByDate.push(result.unit);
   indexedDates[result.date] = arrayByDate;
});
let yourExpectedResult = Object.keys(indexedDates).map(date => ({
  date,
  unit: indexedDates[date].reduce((a, b) => a +b)
}));

let results = [ { unit: 5, date: '2015-12-04 00:01:00' },
      { unit: 10, date: '2015-12-04 00:01:00' },
      { unit: 5, date: '2015-12-04 00:31:00' },
      { unit: 9, date: '2015-12-04 00:31:00' },
      { unit: 5, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:01:00' },
      { unit: 10, date: '2015-12-04 01:31:00' },
      { unit: 5, date: '2015-12-04 01:31:00' },
      { unit: 10, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:01:00' },
      { unit: 5, date: '2015-12-04 02:31:00' },
      { unit: 9, date: '2015-12-04 02:31:00' },
      { unit: 5, date: '2015-12-04 03:01:00' },
      { unit: 9, date: '2015-12-04 03:01:00' },
      { unit: 5, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 03:31:00' },
      { unit: 10, date: '2015-12-04 04:01:00' },
      { unit: 5, date: '2015-12-04 04:01:00' }];
let indexedDates = {};
results.forEach(result => {
   let arrayByDate = indexedDates[result.date] || [];
   arrayByDate.push(result.unit);
   indexedDates[result.date] = arrayByDate;
});
let yourExpectedResult = Object.keys(indexedDates).map(date => ({
  date,
  unit: indexedDates[date].reduce((a, b) => a +b)
}));
console.log(yourExpectedResult);

I think you want this?

const x = results.reduce((accu, r) => {
    const { unit, date } = r;
    accu[date] = accu[date] || { unit: 0, date };
    accu[date].unit += unit;
    return accu;
}, {});
const yourNeed = Object.values(x);

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